FRESNO, Calif. -- The rustic tent cabins of Yosemite National Park – a favorite among families looking to rough it in one of the nation's most majestic settings – have become the scene of a public health crisis after two visitors died from a rodent-borne disease following overnight stays.

On Tuesday, park officials sent letters and emails to 1,700 visitors who stayed in some of the dwellings in June, July and August, warning them that they may have been exposed to the disease that also caused two other people to fall ill.

Those four people contracted hantavirus pulmonary syndrome after spending time in one of the 91 "Signature Tent Cabins" at Curry Village around the same time in June. The illness is spread by contact with rodent feces, urine and saliva, or by inhaling exposed airborne particles.

After the first death, the park sanitized the cabins and alerted the public through the media that the cause might have been diseased mice in the park.

However, officials did not know for sure the death was linked to Yosemite or the campsite until the Centers for Disease Control determined over the weekend that a second visitor, a resident of Pennsylvania, also had died.

After every park tragedy, officials stress that Yosemite is a wilderness area and with it come some dangers.

"We're very concerned about visitors and employees," park spokesman Scott Gediman said. "But we feel we are taking proactive steps in both cleaning the affected areas and in public education. But it's absolutely impossible to eliminate all risk."

On Sunday night, health officials with the National Park Service sent out an alert asking public health authorities to be on the watch for more potential rodent-related cases of acute respiratory failure.

Yosemite receives 4 million tourists a year from around the world, and national park officials were trying to determine if the warning should be expanded to include foreign countries.

"We're discussing whether to do that and how to do that," said Dr. David Wong, chief of the epidemiology branch of the National Park Service Office of Public Health.

The disease can incubate for up to six weeks before flu-like symptoms develop. It's fatal in 30 percent of all cases, and there is no specific treatment. It is not spread human-to-human.

Wong said the Yosemite cases are unusual because hantavirus illnesses are most often isolated events.

"We are seeing more than one person who got it in a narrow space and time," he said. "It makes us wonder why, and those are questions we don't have the answers to."

All the victims stayed in the cabins between June 10 and June 20, and all four known cases were contracted by people who stayed within 100 feet of each other but not necessarily in the same cabins.

The National Park Service currently has assigned two epidemiologists to work in the park trapping rodents for testing. Additional studies are being done to determine if the Yosemite rodent population is higher than normal after a record snowpack in 2011 provided ample water for the grass seeds mice favor.

"Rodents and mice are native to the park, but we are looking at the populations and working with our wildlife biologists to determine if the population is too high," Gediman said. "There are rodents here, and we could never trap them all so that's not going to mitigate it."

As the Labor Day weekend approaches, some people have cancelled reservations at Curry Village after hearing about the outbreak, Gediman said.

The camp sits at the base of the 3,000-foot Glacier Point.

After boulders rained down in 2008, the park permanently closed some cabins. The newer, insulated Signature Cabins were built in 2009 to replace them. Investigators are trying to determine why those cabins were involved in the outbreak.

Rangers are handing out information brochures at the park entrance warning people to avoid mice in general and mouse droppings in particular.

People with reservations in the affected cabins are not being notified before arrival, but they are being warned during check-in to report any sightings of mouse feces.

"This is a serious public health issue and we want to be transparent, but at the same time we don't want people to alter their plans, because we are taking the necessary precautions," Gediman said.

Since the first illness was reported earlier this month, employees of Delaware North disinfected all 408 canvas-sided and wood-sided cabins in Curry Village. Workers are in the midst of shoring up the cabins in an attempt to keep mice from have easy access.

Epidemiologists say none of the victims had anything in common other than staying in Yosemite cabins.

A 37-year-old man from the San Francisco Bay area was the first person to die. Further details have not been released because of medical privacy laws.

Of the 587 documented U.S. cases since the virus was identified in 1993, about one-third proved fatal.

Deer mice were determined to be the main carriers of the virus, though other rodents can be infected, according to the Centers for Disease Control. Most of the cases occur in the West, though researchers are not sure why.

This year's deaths mark the first such fatalities of park visitors, although two others were stricken in a more remote area of the park in 2000 and 2010.

There have been at least two other fatal cases in national parks in the past few years, including a deputy superintendent at Glacier National Park who died in 2004, and a tourist at the Grand Canyon who was stricken in 2009.

Wong said health officials never were able to determine whether the victims contracted hantavirus inside the park grounds.